With the crude oil becoming increasingly heavier and the demand for clean fuel growing, refineries in various counties have implemented multiple sets of large-scale hydrocracking equipment, and expanded the hydrocracking capacity. Before cracking, hydrofining of raw materials is required to remove non-hydrocarbon impurities like sulfur and nitrogen, which is accompanied by reactions such as saturation, ring-opening, dealkylation and isomerization of aromatic hydrocarbons. Therefore, after hydrocracking of crude oil, the tail oil has a saturated hydrocarbon (mainly C20-C30 n-paraffins) content of 96.8% or more and an aromatic content less than 1%, is characterized by low levels of impurities such as sulfur, nitrogen and metal, can directly undergo hydroisomerization, saving the investment and processing costs during pre-processing of raw materials, and is an excellent raw material for producing medium- and low-viscosity group II and group III base oils. Because of the popularity of hydrocracking equipment, using the tail oil as a raw material to produce lubricant base oils has become a mainstream direction.
Hydrocracking tail oil is an optimal raw material to produce API Group III base oil having low viscosity, a low pour point and a viscosity index greater than 120, but has a high solidification point, a high pour point, a high clouding point, partially hydrogenated aromatics, and poor light stability, which necessitates further saturation of the aromatics after iso-dewaxing. The Chevron Corporation is the first manufacturer in the word that used the post-treatment process of hydrocracking-iso-dewaxing-hydrogenation to produce lubricant base oils, which has been widely used.
Currently, hydroisomerization techniques typically use a three-phase reaction (with gas/liquid/solid catalysts), such as the conventional trickle bed technology, to convert n-paraffins to iso-paraffins. In these systems, the continuous phase in the reactor is a gas phase, and a large amount of hydrogen is usually required to maintain the gas phase as continuous in the reactor. This is because, on one hand, the hydroisomerization de-waxing reaction is a slightly exothermic reaction, and in order to maintain the reaction temperature, excessive hydrogen is passed through the catalyst bed to take away the reaction heat; and on the other hand, in the gas-liquid-solid tri-phase reaction, maintaining a high hydrogen partial pressure favors the hydrogenation reaction, inhibits coke formation, and prolongs the life of catalyst. The excess hydrogen is usually pressurized by a hydrogen recycle compressor and then mixed with fresh hydrogen to again serve as the hydrogen feed for the reaction. This process can also be defined as a gas phase recycling trickle bed hydrogenation process. However, under the operation conditions, such a large amount of gaseous hydrogen provided for isomerization increases complexity and expense.
For example, the final effluent from the isomerization reactor is typically separated into a hydrogen-containing gas phase fraction and a liquid fraction, in order to supply and maintain the amount of hydrogen required for a continuous gas phase. The gas phase fraction usually enters a compressor and then recirculates back to the inlet of the reactor to contribute to the supply of a large amount of hydrogen, so as to maintain a continuous gas phase. The hydrogen recycle compressor serves as a key device for the hydrogenation process, the investment thereof represents a large proportion of the total cost of the hydrogenation equipment, and the energy consumption of the hydrogen heat exchanger system is high. If the hydrogen flow in the hydrogenation process can be reduced and the hydrogen circulating system and the hydrogen recycle compressor can be omitted, the cost of investment by enterprises can be saved.
In another aspect, although such three-phase systems typically require a large amount of hydrogen to maintain a continuous gas phase, the hydroisomerization reaction usually does not consume a large amount of hydrogen. In some cases it may consume a certain amount of hydrogen, for example in the isomerization reaction region, where minor cracking may occur. Thus, there is often a large amount of excess hydrogen present throughout the isomerization reaction system, forming a continuous gas phase which is however usually not required for the isomerization reaction. The excess hydrogen is separated from the final effluent, and then further processed by an additional separator and pipe. As discussed above, if this excess hydrogen is recycled to the inlet of the hydroisomerization reaction to supply hydrogen to the system, the hydrogen must be supplied to the reactor at a desired high pressure by going through a high pressure compressor.
The two-phase hydrogenation process (e.g., liquid materials and solid catalysts) is also proposed to, in some cases, convert certain hydrocarbon-containing materials to other hydrocarbons more valuable (liquid phase reactors can be used in this process). For example, by pre-saturation of hydrogen, two-phase systems (liquid phase reactors), rather than conventional three-phase systems, can be used to reduce sulfur in some hydrocarbon streams.
Other uses of liquid phase reactors are for hydrocracking and hydrotreatment of hydrocarbon-containing materials. However, hydrotreatment and hydrocracking require a significant amount of hydrogen to perform corresponding chemical conversions. Thus, even if all of these reactions proceed in a liquid phase system, a large amount of hydrogen is still required. Therefore, in order to maintain the hydrogen required for such a liquid phase hydrotreatment or hydrocracking reaction, it is necessary to introduce an additional diluent or solvent into the raw material of the existing liquid phase system to dilute the reaction components in the feed and reduce the temperature rise in the reactor. Thus, the diluent and the solvent should have greater hydrogen solubility than the raw materials, to ensure sufficient conversion in the liquid phase. However, these reaction systems often require larger, more complex, and more expensive liquid phase reactors to achieve a desired conversion.
Currently, the two-phase processes mainly include the IsoTherming technology of DuPont. The U.S. Pat. No. 6,881,326B2 and ZL200680018017.3 of the company disclose use of diluents or solvents to provide greater hydrogen solubility, wherein a product is used as the diluent or solvent. However, because of hydrocracking or hydrotreatment reactions, hydrogen consumption is large, resulting in a large throughput of product circulation.
U.S. Pat. No. 7,803,269B2 discloses a liquid-phase hydroisomerization process for hydroisomerization of Fischer-Tropsch synthetic oils or vegetable oils consisting of C8-C30 linear paraffins, so as to reduce the clouding point, pour point and solidification point. The hydroisomerization process is characterized by having low hydrogen consumption, avoiding product circulating, and not requiring supplementation of additional hydrogen in the hydroisomerization isomerization region. However, the process is not suitable for the hydroisomerization of mineral oil fractions or synthetic oils having a high dry point and/or a high aromatic content. Moreover, the process is not capable of hydrofining the hydroisomerized oil and cannot further saturate aromatics. Therefore, in order to improve the color and oxidation stability of the product, additional additives are required.